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<pre><span class="sourceLineNo">001</span><a name="line.1"></a>
<span class="sourceLineNo">002</span>/*<a name="line.2"></a>
<span class="sourceLineNo">003</span> * Copyright (C) 2011 Archie L. Cobbs. All rights reserved.<a name="line.3"></a>
<span class="sourceLineNo">004</span> *<a name="line.4"></a>
<span class="sourceLineNo">005</span> * $Id: ObjectComparator.html 947 2014-11-12 22:14:09Z archie.cobbs $<a name="line.5"></a>
<span class="sourceLineNo">006</span> */<a name="line.6"></a>
<span class="sourceLineNo">007</span><a name="line.7"></a>
<span class="sourceLineNo">008</span>package org.dellroad.stuff.java;<a name="line.8"></a>
<span class="sourceLineNo">009</span><a name="line.9"></a>
<span class="sourceLineNo">010</span>import com.google.common.cache.CacheBuilder;<a name="line.10"></a>
<span class="sourceLineNo">011</span>import com.google.common.cache.CacheLoader;<a name="line.11"></a>
<span class="sourceLineNo">012</span>import com.google.common.cache.LoadingCache;<a name="line.12"></a>
<span class="sourceLineNo">013</span><a name="line.13"></a>
<span class="sourceLineNo">014</span>import java.util.Comparator;<a name="line.14"></a>
<span class="sourceLineNo">015</span>import java.util.concurrent.atomic.AtomicLong;<a name="line.15"></a>
<span class="sourceLineNo">016</span><a name="line.16"></a>
<span class="sourceLineNo">017</span>/**<a name="line.17"></a>
<span class="sourceLineNo">018</span> * A {@link Comparator} that creates a stable, total ordering of all Java objects.<a name="line.18"></a>
<span class="sourceLineNo">019</span> *<a name="line.19"></a>
<span class="sourceLineNo">020</span> * &lt;p&gt;<a name="line.20"></a>
<span class="sourceLineNo">021</span> * Instances are compared first using {@linkplain System#identityHashCode identity hash codes}, and then using<a name="line.21"></a>
<span class="sourceLineNo">022</span> * an internally generated unique identifier; identifiers are indexed using weak keys to avoid memory leaks.<a name="line.22"></a>
<span class="sourceLineNo">023</span> * Any two distict Java objects will always compare as non-equal.<a name="line.23"></a>
<span class="sourceLineNo">024</span> * &lt;/p&gt;<a name="line.24"></a>
<span class="sourceLineNo">025</span> *<a name="line.25"></a>
<span class="sourceLineNo">026</span> * &lt;p&gt;<a name="line.26"></a>
<span class="sourceLineNo">027</span> * Note: this ordering is only &lt;i&gt;consistent with equals&lt;/i&gt; (see {@link Comparable}) for classes whose<a name="line.27"></a>
<span class="sourceLineNo">028</span> * {@link #equals equals()} method is implemented in terms of object equality (e.g., not overridden at all).<a name="line.28"></a>
<span class="sourceLineNo">029</span> * &lt;/p&gt;<a name="line.29"></a>
<span class="sourceLineNo">030</span> *<a name="line.30"></a>
<span class="sourceLineNo">031</span> * &lt;p&gt;<a name="line.31"></a>
<span class="sourceLineNo">032</span> * Note: while each instance creates a stable sort ordering, distinct instances of this class may sort objects differently.<a name="line.32"></a>
<span class="sourceLineNo">033</span> * &lt;/p&gt;<a name="line.33"></a>
<span class="sourceLineNo">034</span> *<a name="line.34"></a>
<span class="sourceLineNo">035</span> * &lt;p&gt;<a name="line.35"></a>
<span class="sourceLineNo">036</span> * Null values are supported and always sort last. Instances of this class are thread safe.<a name="line.36"></a>
<span class="sourceLineNo">037</span> * &lt;/p&gt;<a name="line.37"></a>
<span class="sourceLineNo">038</span> *<a name="line.38"></a>
<span class="sourceLineNo">039</span> * &lt;p&gt;<a name="line.39"></a>
<span class="sourceLineNo">040</span> * This class requires &lt;a href="https://github.com/google/guava"&gt;Google Guava&lt;/a&gt;.<a name="line.40"></a>
<span class="sourceLineNo">041</span> * &lt;/p&gt;<a name="line.41"></a>
<span class="sourceLineNo">042</span> */<a name="line.42"></a>
<span class="sourceLineNo">043</span>public class ObjectComparator implements Comparator&lt;Object&gt; {<a name="line.43"></a>
<span class="sourceLineNo">044</span><a name="line.44"></a>
<span class="sourceLineNo">045</span>    private final AtomicLong nextId = new AtomicLong();<a name="line.45"></a>
<span class="sourceLineNo">046</span>    private final LoadingCache&lt;Object, Long&gt; idMap;<a name="line.46"></a>
<span class="sourceLineNo">047</span><a name="line.47"></a>
<span class="sourceLineNo">048</span>    /**<a name="line.48"></a>
<span class="sourceLineNo">049</span>     * Default constructor.<a name="line.49"></a>
<span class="sourceLineNo">050</span>     */<a name="line.50"></a>
<span class="sourceLineNo">051</span>    public ObjectComparator() {<a name="line.51"></a>
<span class="sourceLineNo">052</span>        this(CacheBuilder.newBuilder());<a name="line.52"></a>
<span class="sourceLineNo">053</span>    }<a name="line.53"></a>
<span class="sourceLineNo">054</span><a name="line.54"></a>
<span class="sourceLineNo">055</span>    /**<a name="line.55"></a>
<span class="sourceLineNo">056</span>     * Constructor.<a name="line.56"></a>
<span class="sourceLineNo">057</span>     *<a name="line.57"></a>
<span class="sourceLineNo">058</span>     * @param concurrencyLevel guides the allowed concurrency among update operations; used as a hint for internal sizing<a name="line.58"></a>
<span class="sourceLineNo">059</span>     * @throws IllegalArgumentException if {@code concurrencyLevel} is zero or less<a name="line.59"></a>
<span class="sourceLineNo">060</span>     * @see CacheBuilder#concurrencyLevel<a name="line.60"></a>
<span class="sourceLineNo">061</span>     */<a name="line.61"></a>
<span class="sourceLineNo">062</span>    public ObjectComparator(int concurrencyLevel) {<a name="line.62"></a>
<span class="sourceLineNo">063</span>        this(CacheBuilder.newBuilder().concurrencyLevel(concurrencyLevel));<a name="line.63"></a>
<span class="sourceLineNo">064</span>    }<a name="line.64"></a>
<span class="sourceLineNo">065</span><a name="line.65"></a>
<span class="sourceLineNo">066</span>    /**<a name="line.66"></a>
<span class="sourceLineNo">067</span>     * Internal constructor.<a name="line.67"></a>
<span class="sourceLineNo">068</span>     *<a name="line.68"></a>
<span class="sourceLineNo">069</span>     * @param cacheBuilder object unique ID cache builder<a name="line.69"></a>
<span class="sourceLineNo">070</span>     * @throws NullPointerException if {@code cacheBuilder} is null<a name="line.70"></a>
<span class="sourceLineNo">071</span>     */<a name="line.71"></a>
<span class="sourceLineNo">072</span>    protected ObjectComparator(CacheBuilder&lt;Object, Object&gt; cacheBuilder) {<a name="line.72"></a>
<span class="sourceLineNo">073</span>        this.idMap = cacheBuilder.weakKeys().&lt;Object, Long&gt;build(new CacheLoader&lt;Object, Long&gt;() {<a name="line.73"></a>
<span class="sourceLineNo">074</span>            @Override<a name="line.74"></a>
<span class="sourceLineNo">075</span>            public Long load(Object obj) {<a name="line.75"></a>
<span class="sourceLineNo">076</span>                return ObjectComparator.this.nextId.getAndIncrement();<a name="line.76"></a>
<span class="sourceLineNo">077</span>            }<a name="line.77"></a>
<span class="sourceLineNo">078</span>        });<a name="line.78"></a>
<span class="sourceLineNo">079</span>    }<a name="line.79"></a>
<span class="sourceLineNo">080</span><a name="line.80"></a>
<span class="sourceLineNo">081</span>    @Override<a name="line.81"></a>
<span class="sourceLineNo">082</span>    public int compare(Object obj1, Object obj2) {<a name="line.82"></a>
<span class="sourceLineNo">083</span><a name="line.83"></a>
<span class="sourceLineNo">084</span>        // Handle the equality case first to avoid creating unique ID's for no reason<a name="line.84"></a>
<span class="sourceLineNo">085</span>        if (obj1 == obj2)<a name="line.85"></a>
<span class="sourceLineNo">086</span>            return 0;<a name="line.86"></a>
<span class="sourceLineNo">087</span><a name="line.87"></a>
<span class="sourceLineNo">088</span>        // Handle one of the objects being null<a name="line.88"></a>
<span class="sourceLineNo">089</span>        if (obj1 == null)<a name="line.89"></a>
<span class="sourceLineNo">090</span>            return 1;<a name="line.90"></a>
<span class="sourceLineNo">091</span>        if (obj2 == null)<a name="line.91"></a>
<span class="sourceLineNo">092</span>            return -1;<a name="line.92"></a>
<span class="sourceLineNo">093</span><a name="line.93"></a>
<span class="sourceLineNo">094</span>        // Compare hash values<a name="line.94"></a>
<span class="sourceLineNo">095</span>        final int hash1 = System.identityHashCode(obj1);<a name="line.95"></a>
<span class="sourceLineNo">096</span>        final int hash2 = System.identityHashCode(obj2);<a name="line.96"></a>
<span class="sourceLineNo">097</span>        if (hash1 &lt; hash2)<a name="line.97"></a>
<span class="sourceLineNo">098</span>            return -1;<a name="line.98"></a>
<span class="sourceLineNo">099</span>        if (hash1 &gt; hash2)<a name="line.99"></a>
<span class="sourceLineNo">100</span>            return 1;<a name="line.100"></a>
<span class="sourceLineNo">101</span><a name="line.101"></a>
<span class="sourceLineNo">102</span>        // Compare unique serial numbers (it should be very rare that we get to this point)<a name="line.102"></a>
<span class="sourceLineNo">103</span>        final long id1 = this.idMap.getUnchecked(obj1);<a name="line.103"></a>
<span class="sourceLineNo">104</span>        final long id2 = this.idMap.getUnchecked(obj2);<a name="line.104"></a>
<span class="sourceLineNo">105</span>        assert id1 != id2;<a name="line.105"></a>
<span class="sourceLineNo">106</span>        return Long.compare(id1, id2);<a name="line.106"></a>
<span class="sourceLineNo">107</span>    }<a name="line.107"></a>
<span class="sourceLineNo">108</span>}<a name="line.108"></a>
<span class="sourceLineNo">109</span><a name="line.109"></a>




























































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